<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <script src="./vue.js"></script>
</head>
<body>
  <div id="app" class="demo-vm-1">
    <p>{{arr[0]}}</p>
    <p>{{arr[2]}}</p>
    <p>{{arr[3].c}}</p>
  </div>
</body>

<script>
  new Vue({
    el: "#app",
    data() {
      return {
        arr: [1, 2, { a: 3 },{ c: 5}]
      }
    },
    mounted() {
      console.log("demo Instance: ", this.$data);

      setTimeout(() => {
        console.log('update')
        this.arr[0] = { o: 1 }
        this.arr[2] = { a: 1 }
      },2000)
    },
  })
</script>
</html>